The following patent publications are believed to represent the current state of the art:
U.S. Patent Publication Nos. US2007/156436; US2009/104888; US2009/0313105; US2010/0241494; US2010/049615; US2010/088188; US2010/274677; US2011/191196; US2011/264543; US2011/320291; US2012/004972 and US2012005026;
Published PCT Application Nos. WO2009129749; WO2010062077; WO2011094424; WO2011089423 and WO2011112158; and
Patent Publication Nos. GB2480282; EP2372628; CN102256001; CN101567108A; TWM416824Y and KR20090121497A.